1. Field of the Invention
The invention relates to a nitride semiconductor wafer.
2. Description of the Related Art
When a device structure with a heterojunction, such as high-electron-mobility transistor (HEMT), is formed on a (111) plane or a C plane [(0001) plane] of a conductive or semi-insulating semiconductor substrate formed of Si, SiC or GaN, etc., a buffer layer is required to have high insulating properties. However, sufficient insulating properties are not necessarily obtained in case of using a single-layered buffer layer since an energy barrier is not present in the buffer layer and free electrons injected into the buffer layer thus move easily.
In order to improve insulating properties, use of a buffer layer including a heterojunction formed between, e.g., an aluminum gallium nitride (AlGaN) layer and a gallium nitride (GaN) layer is effective but an induced charge, which is caused by a polarization effect characteristic of the nitride semiconductor, impedes improvement in insulating properties. In an AlGaN/GaN heterojunction structure stacked in a c-axis direction, free carriers of opposite sign are likely to be induced at an interface due to spontaneous and piezo polarizations inherent in nitride-based semiconductor crystal. In the buffer layer including a heterojunction as described above, a polarization moment depends on an Al composition x of an AlxGa1-xN layer and polarization charge density thus depends on the Al composition x (see, e.g., Non-patent literature: Journal of Applied Physics, Vol. 87, page 334 (2000) by O Ambacher et al). According to Non-patent literature, a charge σPsp/e, which is equivalent to a difference between spontaneous polarization inherently present in the AlGaN layer and that inherently present in the GaN layer, is induced at an interface between the AlGaN layer and the GaN layer. In addition, a charge σPpe/e, which is equivalent to a difference between piezo polarization in the AlGaN layer caused by elastic stress generated in the AlGaN layer in response to an external force and that in the GaN layer caused by elastic stress generated in the GaN layer in response to an external force, is induced at the interface between the AlGaN layer and the GaN layer. Accordingly, σ/e (Psp+Ppe) as the total of σPsp/e and σPpe/e is equivalent to the total polarization charge induced at the interface between the AlGaN layer and the GaN layer.
Then, when a relation between the Al composition x in the composition formula AlxGa1-xN of the AlGaN layer and ρ/e (Psp+Ppe) is derived by an experiment, a curve representing this relation is expressed by a relational expression: σ/e (Psp+Ppe)=5×1013·x (x≦0.6), σ/e (Psp+Ppe)=1×1014·x−3×1013 (x>0.6).
That is, according to Non-patent literature, when, in the buffer layer having an AlGaN/GaN heterojunction structure, the composition formula of the AlGaN layer is defined as AlxGa1-xN and polarization charge density at each interface between the AlGaN layer and the GaN layer in the buffer layer is defined as Np, the relation is expressed by approximately Np=5×1013·x (cm−2) when x≦0.6 and approximately Np=1×1014·x−3×1013 (cm−2) when x>0.6. When the Al composition x is, e.g., 0.1, the polarization charge density Np is approximately 5×1012 (cm−2).
The polarization charge induces a charge of opposite sign at both ends of a layer but is electrically neutral when the AlGaN/GaN heterojunction structure is independent, hence does not cause any problems. However, once an active layer such as a HEMT structure is formed on an upper layer of the heterojunction structure, it causes variation in a band diagram and the neutrality conditions are partially unsatisfied. In a HEMT epi structure which is a monopolar device having free electrons as a carrier, electrons are likely to be preferentially induced also at an AlGaN/GaN interface in the buffer layer. In order to suppress this, introduction of an acceptor into the buffer layer is considered to be effective.
A compound semiconductor substrate is known in which carbon is doped into the entirety of a multilayer buffer layer as an alternating layer formed of two types of aluminum gallium nitride layers having different compositions (see, e.g., JP-A-2011-82494). According to JP-A-2011-82494, the withstand voltage performance of a device can be improved by configuring the multilayer buffer layer to contain carbon.